Micromechanically active reconfigurable add-drop filters

Details Micromechanically active reconfigurable add-drop filters Micromechanically active reconfigurable add-drop filters

2001-05-21

2004-06-15

Stafira, Michael P. (Department: 2877)

Optical waveguides

With optical coupler

Input/output coupler

C385S015000

Reexamination Certificate

active

06751377

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to optical add-drop filters and, in particular, to micromechanically active, reconfigurable add-drop filters.
BACKGROUND OF THE INVENTION
Optical communication systems are beginning to achieve their great potential for the rapid transmission of vast amounts of information. In essence, an optical communication system comprises a source of light, a modulator for impressing information on the light to produce optical signals, an optical fiber transmission line for carrying the optical signals and a receiver for detecting the signals and demodulating the information they carry. Increasingly the optical signals are wavelength division multiplexed signals (WDM signals) comprising a plurality of distinct wavelength signal channels.
Add/drop devices are important components of WDM optical communication systems. Such devices are typically disposed at various intermediate points along the transmission fiber (called nodes) to permit adding or dropping of signal channels at the nodes. Thus, for illustration, an add/drop device would permit a transmission line from New York to Los Angeles to drop off at Chicago signal channels intended for Chicago and to add at Chicago signal channels for New York and Los Angeles. As the number of nodes increases, the number of add/drop devices increases, and their cost and effect on the system become appreciable.
FIG. 1
schematically illustrates a conventional optical add-drop filter
10
known as a microring add-drop filter. The filter
10
comprises, in essence, a pair of optical waveguides
11
and
12
optically coupled by a microscale resonator
13
comprising a waveguide ring closely adjacent each of the wave guides
11
,
12
. The ring
13
is optically resonant for optical wavelengths &lgr;
i
such that n&lgr;
i
=C, where C is the circumference of the ring and n is an integer.
In operation, if a set of wavelengths &lgr;
1
, &lgr;
2
, . . . &lgr;
N
is incident on input port I of waveguide
11
, any of the wavelengths resonant with the microring resonator will couple across the resonator
13
to waveguide
12
and exit the filter
10
at drop port R. Nonresonant wavelengths will pass the ring structure unperturbed and exit the filter
10
at the through port T. In addition, resonant wavelengths can be added at the add port A and will exit at port T.
The diameter D of the ring is chosen sufficiently small to obtain a desired free spectral range. To obtain a free spectral range of the order of tens of nanometers, D must be less than about 10 micrometers. With such small diameters, the index contrast between the ring and its cladding (the lateral index contrast) must be high to avoid bending losses. Typically, the rings are fabricated with air cladding in the lateral direction.
In view of the high lateral index contrast, the coupling distances d
1
and d
2
between the ring
13
and waveguides
11
,
12
, respectfully, must be small—typically less than 300 nanometers in order to obtain the necessary coupling. In alternative embodiments, the microring resonator
13
can be replaced by a microdisk resonating in whispering gallery modes. Further details concerning the structure and operation of conventional microring and microdisk add-drop filters are set forth in B. E. Little, et al, “Microring Resonator Channel Dropping Filters”, 15
Journal of Lightwave Technology
998 (1997); B. E. Little, et al., “Ultracompact Si—SiO
2
Microring Resonator Optical Channel Dropping Filters, 10
IEEE Photonics Technology Letters
549 (1998); and D. Radfizadeh, et al., “Wave-Guide-Coupled AlGaAs/GaAs Microcavity Ring and Disk Resonators . . . ”, 22
Optics Letters
1244 (1997), each of which is incorporated herein by reference.
While theoretically promising, microring and microdisk add-drop filters are difficult to fabricate with necessary precision. For example, a good quality add-drop filter must essentially eliminate a dropped wavelength so that it does not reach the port T. (The filter must achieve a high extinction ratio for the dropped wavelength.) This elimination requires precise control of the coupling distances d
1
, d
2
. But due to their small sizes (less than 300 nm), these distances are difficult to fabricate with the necessary precision. Published results to date have shown only slightly better than 10 dB extinction for the best individual devices.
Another challenge in fabrication is to make microrings or microdisks with precise resonant frequencies. An add-drop filter for telecommunications would need rings or disks with diameters specified and fabricated to better than 1 part in 1500 in order to overlap a dense WDM grid (100 GHz spacing). Moreover, sidewall roughness of the ring adds a further degree of uncertainty to the precise value of the diameter.
Finally it should be noted that the conventional microring and microdisk add-drop filters are fixed in configuration. Once fabricated, the filter will always add and drop the same respective wavelengths. However, in contemplated systems it would be highly advantageous if add-drop filters could be dynamically reconfigured to select and change which wavelength channels are added and dropped.
SUMMARY OF THE INVENTION
In accordance with the invention, a tunable, reconfigurable optical add-drop filter comprises a pair of optical waveguides optically coupled by a microring or microdisk resonator wherein the coupling distance between the resonator and at least one of the waveguides is micromechanically controllable. With this arrangement, the degree of coupling can be tuned after fabrication to provide high level extinction of dropped wavelengths and the filter can be dynamically reconfigured. Advantageously, laser radiation is provided to tune the resonant wavelength.


REFERENCES:
patent: 5905573 (1999-05-01), Stallard et al.
patent: 6195187 (2001-02-01), Soref et al.
patent: 6473541 (2002-10-01), Ho
Hagness, et al., “FDTD Microcavity Simulations: Design and Experimental Realization of Waveguide-Coupled Single-Mode Ring and Whispering-Gallery-Mode Disk Resonators”, Journal of Lightwave Technology, vol. 15, No. 11, Nov. 1997.
DeLong, K.W. et al, “Effect of two-photon absorption on all-optical guided-wave devices”, Appl. Phys. Lett. 55(18), Oct. 30, 1989.
Madsen, Christi K. and Zhao, Jian H., “Optical Filter Design and Analysis”, Wiley-Interscience Publication, John Wiley & Sons, Inc., pp. 68-77 New York, 1999.
Blair, Steve, et al., “Beyond the absorption-limited nonlinear phase shift with microring resonators”, Optical Society of America, Optics Letters, vol. 27, No. 5, Mar. 1, 2002.

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